1. Field of the Invention
The present invention relates to a short gate high power MOSFET, and a method of making a short gate high power UMOSFET.
2. Description of the Background Art
In silicon carbide (SiC) MOSFETs (metal oxide semiconductor field effect transistor), inversion channel mobility is dramatically lower than in silicon based MOSFETs. The relatively poor inversion channel mobility is due in large part to the gate oxidation process, whereby a rough interface between the gate oxide and the underlying silicon carbide is formed. Defects which occur at the gate oxide/silicon carbide interface due to the rough interface reduce channel mobility.
A common approach to improving inversion channel mobility of silicon carbide MOSFETs focuses on reducing channel length, so that the distance traveled by carriers within the inversion channel underneath the gate is as short as possible. However, a problem encountered with this approach is that as channel length is reduced, breakdown voltage of the MOSFET device becomes limited. If the channel length is too short, the channel may open responsive to a high voltage applied to the drain even without a necessary voltage applied to the gate, to thus force the device into an on state when it should be off. This is commonly referred to as a short-channel effect, or an early turn-on effect. A need thus exists to protect the channel of the device from excessive drain voltage, so as to prevent short channel effects such as the early turn-on effect. Key aspects of short channel MOSFET design thus include limiting the impact of low channel mobility by reducing the length of the conduction path at the MOS interface, while at the same time preventing short-channel effects.